How covers inpact the inductance, and then some...

[Peter Naglitsch]

OK, you think that you have got the hang on things and then the real world catches up on you?

I have always been under the impression that more metal in the pickup = higher inductance, and high inductance = muddy top end. Now I have a set of fresh ones here (double slug bobbins, lot of metal in the coil) and finally a LCR meter. So I measure the Inductance without covers 9.23 H (@250Hz) and with covers 8.35H. The inductance drops 10% with covers on. Repeated that with an experimental set and although the nubers differt quite radical the tendency was the same. The cover made the inductance go down. I tried exchanging the ceramic magnet from the first set with an ALNICO one and the inductance went up to 9.86 = +7% (without cover). I thought that more or less all metal within the magnetic field boosts the inductance. Or is it only metal in the ?magnetic circuit? that boost the inductance? Or what?

And have I at all understood the connection between inductance and tone (high inductance = muddy top end)?

I really would appreciate some input on this.

[Steve Conner]

That's exactly what you'd expect. The cover acts as a shorted turn, so it lowers the inductance and lowers the Q of the resonant peak somewhat. Because it's non-magnetic metal, it doesn't have any effect on the magnetic circuit that would increase the inductance. The rule is that non-ferrous metals lower the inductance and ferrous ones increase it.

High inductance can be muddy, or if combined with a big resonant peak, it can honk and roar too. High output distortion humbuckers like the Duncan Invader sound real muddy and honky when you play them clean, but with high gain, the tone just works. The resonant peak colours the sound almost like a parked wah, and the distortion puts back the high-end harmonics that the high inductance filtered out.

I guess if you put a cover on an Invader, it would just sound dull.

[Possum]

....

Yeah, covers and baseplates make inductance go down. But pay close attention to the AC resistance, watch it go UP when you put that cover on, thats where you see the treble killing effect, not in the inductance. Try it with brass cover then nickel silver, brass is a real tone killer in covers.....

[cbarrow7625]

But pay close attention to the AC resistance, watch it go UP when you put that cover on, thats where you see the treble killing effect, not in the inductance..

Possum, I assume that by "AC Resistance" you mean the AC Impedance. (I have zero experience building pickups so I'm asking this question with a complete open mind). But usually when inductance goes down, the AC impedance at higher frequencies usually goes down as well, no?

I have a lot of experience building speaker crossovers. Certainly in that circumstance when a lower value series inductor is used the Impedance is reduced at higher frequencies because the low-pass effect of the inductor begins at a higher frequency, therefore the rising impedance also starts at a higher frequency. As a result, the impedance at higher frequencies is typically lower with a small inductor than a large inductor.

Just curious how your experience with pickup High Frequencies relates to this. As you can read, I was a little confused by your statement.

Chris

[Mike Sulzer]

OK, you think that you have got the hang on things and then the real world catches up on you?

... a LCR meter. So I measure the Inductance without covers 9.23 H (@250Hz) and with covers 8.35H. (@250Hz)

Does your LCR meter measure low Q inductors with good accuracy? The inductance reactance of 9.23 H at 250 Hz is about 15,000 ohms. The resistance of your pickup is, maybe, 8000 ohms? This is a substantial fraction of the reactance, possibility leading to errors in the measurement. Additional losses introduced by conductors could also contribute to the error.

[soundmasterg]

There is a reason mostly everyone uses the Extech 380193 meter and that is because it is reasonably accurate with low Q inductors among other things. Even Rickenbacker uses these meters.........

Greg

[David Schwab]

I would think the metal needed to affect the inductance would have to be ferromagnetic material. There is some nickel in nickel silver, but not a whole lot.

Possum, I assume that by "AC Resistance" you mean the AC Impedance.

Impedance is resistance to AC. But you take readings of the AC resistance. The AC resistance of a pickup increases with frequency.

[Joe Gwinn]

Possum, I assume that by "AC Resistance" you mean the AC Impedance. (I have zero experience building pickups so I'm asking this question with a complete open mind). But usually when inductance goes down, the AC impedance at higher frequencies usually goes down as well, no?

No, it really is a resistance (in phase), not reactance (quadrature). The AC resistance of a pickup is basically the DC resistance of the coil plus the eddy-current losses.

I have a lot of experience building speaker crossovers. Certainly in that circumstance when a lower value series inductor is used the Impedance is reduced at higher frequencies because the low-pass effect of the inductor begins at a higher frequency, therefore the rising impedance also starts at a higher frequency. As a result, the impedance at higher frequencies is typically lower with a small inductor than a large inductor.

The thing to understand about pickups is that their Q is quite low, so they are very impure inductances.

[Mike Sulzer]

This setup can be used to measure the relative impedance of a pickup versus frequency:

noise generator ==> 1 meg resistor ==> preamp ==> FFT analyzer

The measured voltage versus frequency is the relative impedance. The pickup used for the test was 11K, #43, slugs one side , screws the otherside, no magnet. (That would make some difference.) The test was made for three conditions: 1. no cover; 2, cover on; 3. Cover sitting upsidedown on the pickup so that the top is covered, but the sides are not.

The results are shown here: http://www.naic.edu/~sulzer/coverEffects.png
The results are the same for all three cases at the high and low frequency limits. Since the high frequency part of the curves show the capacitance, the capacitances of all three are the same to within the measurement accuracy, as expected. Since the low frequency part of the curves show the inductance, the inductances are effectively the same. The peaks are different, and so the Qs are different. The loses are lower when the cover is off. The other two cases are nearly the same. This implies that it is primarily the top plate of the cover that is causing the losses. This is expected for a humbucker since the two coils are wound in opposite directions and so the total enclosed flux around both tends to cancel. A similar effect has been discussed on the thread: Brass and eddy currents.

[Mike Sulzer]

I forgot to say that I added capacitance across the pickup to bring the resonance down to about 2 KHz. Like a very long cable.

[RedHouse]

... when you put that cover on, thats where you see the treble killing effect....

I just don't get "treble killing" at all, in my winds, sure one can detect a slight variation in presence (maybe upper treble or "silibance" area in engineering speak) but it's way far and away from being "treble killing".

I wonder what you are doing that I'm not, or vise versa, to experience treble killing from pickup covers. Are you using decent nickel covers?

[Mike Sulzer]

I just don't get "treble killing" at all, in my winds, sure one can detect a slight variation in presence (maybe upper treble or "silibance" area in engineering speak) but it's way far and away from being "treble killing".

I wonder what you are doing that I'm not, or vise versa, to experience treble killing from pickup covers. Are you using decent nickel covers?

The effect really is not very big, as you can see from the measurement. Some people attribute more importance to it than others.

[greenfingers]

a kid on youtube posted this, and it shows the tone change when removing the covers. http://youtube.com/watch?v=eZO5Olq5tqk

[RedHouse]

a kid on youtube posted this, and it shows the tone change when removing the covers. http://youtube.com/watch?v=eZO5Olq5tqk

Ya know I wonder if that video was done on one of those Epi Les Paul's, I have had some experience with some of those (HB6N and HB6B) Epi pickups from my friend's, son's, Epi Lester which exibited that kind of treble-loss, thing but I always just attributed it to the uber-cheap pickup parts (ie; magnet weak, stud poles barely magnetic, nickel plated thick brass covers etc).

Lot's of noticable difference there to be sure, that's not the result I get when doing that, but then I do use good parts and a decent nicklel cover.

[David Schwab]

Lot's of noticable difference there to be sure, that's not the result I get when doing that, but then I do use good parts and a decent nicklel cover.

With good covers it's subtle. But as I've said before, look how people started taking the covers off back in the 70's. It wasn't just for looks. There were no hot pickups back then, so people took the covers off to get a bit more top and and output. I even took them off my Sekova Les Paul Custom copy. It had those funky rectangular clear nylon bobbins like in mini hums.

People putting the covers back on now is part of the retro "vintage" fad, as far as I'm concerned. You can't have rusty worn looking covers if you have no covers! But that's the image people have in their mind of old vintage (not well cared for) guitars.